Conventionally, devices using a cathode ray tube (CRT) have been used extensively as display units. The CRT requires an installation space of a prescribed size or more, so that the liquid crystal display is rapidly wide spreading as a light and thin display unit. The liquid crystal display is being used for displays of mobile phones, PDAs and the like; monitors for personal computers; home televisions and other various types of home appliances. And, the plasma display panel (PDP) has been put to practical use as a self-luminous type display device. A display unit using an electron emission element such as a field emission type cold cathode, namely a field emission type display (FED), is also being put into practical use.
The above-described display devices are first required to be easily readable as a matter of course. Therefore, it is necessary to suppress surface reflection on the screen in order to prevent a background, which degrades a contrast, from reflecting on the screen. Accordingly, the surface of the display device is generally treated for antireflection. An antireflection film is formed by alternately stacking thin films having high and low different refractive indexes according to an optical design, thereby causing interference of the reflected lights to attenuate the reflectance. As a method of forming such an antireflection film, a vapor deposition method or a sol-gel method has been mainly adopted, but a sputtering method is began to be adopted in view of production capacity and controllability of film thickness lately.
For component materials of the antireflection film, an oxide film of Ta, Nb, Ti, Zr, Hf or the like is mainly used as a high refractive index film, and an oxide film of Si is mainly used as a low refractive index film (e.g., Patent Documents 1 through 3). Known methods of forming an Si oxide film include (1) a method of performing reactive sputter by using an Si target in a mixture gas atmosphere of Ar and O2, (2) a method of performing plasma oxidation of an Si film which is formed by using an Si target, and (3) a method of performing sputter film formation by using a SiOx target. It is general to use the film forming methods (1) and (2), and a monocrystalline or polycrystalline Si target which is produced by a melting method is used.
Incidentally, a refractive index is particularly important for an optical thin film such as a thin film for an antireflection film. In addition, the display devices adopting the antireflection film and also the fields of portable phones, automobiles, building materials and the like require mass production, so that thin films having the same levels of refractive index are required to be formed efficiently and stably. Here, the refractive index of the thin film highly depends on its thickness, so that the stability of the film thickness must be enhanced in a step of forming the thin film for an antireflection film by the sputtering method. Especially, it is important that the thickness of the thin film is prevented from varying until about the end of the life of the sputtering target.
In a step of forming an Si oxide film by using a conventional Si target, a film-forming speed is controlled in order to control the thickness of a thin film. But, when an Si target formed of a conventional melting material is used, there is a tendency that the film-forming speed decreases as the life end comes closer. The film thickness distribution in the surface is varied with a change in the film-forming speed. The refractive index of the thin film highly depends on its thickness, so that the uniformity of a reflection preventing effect is deteriorated if the in-plane distribution of the thickness is varied. Thus, the Si oxide film, which is formed by using the conventional Si target, causes degradation of the characteristics, nonuniformization or the like of the antireflection film.
Besides, the Si target formed of a conventional melting material has a problem that the Si oxide film has a low film forming speed. The film forming speed has a direct effect on the Si oxide film and on the production cost of the antireflection film. The Si target formed of the conventional melting material also has a problem that it is easily cracked in a stage of a low frequency of use, e.g., a stage in that an erosion depth has not reached 1/4 of the thickness of the Si target. Thus, the slow film forming speed and the early crack of the Si target are causes of increasing the film forming cost of the Si oxide film and a defect occurrence rate.
Patent Document 1: Pamphlet of (PCT) International Publication No. WO 97/08359
Patent Document 2: Japanese Patent Laid-Open Application No. HEI 11-171596 A1
Patent Document 3: Japanese Patent Laid-Open Application No. 2002-338354 A1